The present invention relates to a device for taking liquid samples, permitting in particular a good homogenization of the liquid to be analysed, as well as to a sampling bench using such a device.
In nuclear installations, when it is desired to take radioactive liquid samples for analysis purposes, the standard method consists of using a hollow needle, connected to a storage tank of the liquid to be analysed and on which is pierced a container or jug, provided with an elastic plug. As this container has been previously placed under vacuum, it automatically fills with the liquid by suction. The vessels containing the liquids to be sampled are located under protective means, at distances of often several dozen meters, so that it is necessary to raise the liquid up to the location where sampling is to take place. Various types of devices have been used hitherto, as a function of the distance between the sampling point and the vessel containing the liquid to be analysed.
In the case of short distances (2 to 5 m) a needle, e.g. having an internal diameter of 1.2 mm and a thickness of 0.5 mm, directly connects the sampling point to the vessel containing the liquid to be sampled. No homogenization of the liquid is possible in such a device, which is obviously prejudicial to the presentativeness of the analysis. In addition, deposits can occur within the needle and the raised liquid can dissolve the deposits which occur during the previous sampling operation, which further falsifies the measurements.
For greater distances and level differences, the liquid is raised into intermediate tanks by vacuum action. The closest tank to the sampling point has a small volume cavity in which is located the end of the needle. It is possible to perform a rinsing of the installation up to the level of the cavity by a sequence of raising and draining operations, but level with the needle, it is not always possible to carry out an adequate homogenization and prevent unwanted deposits.
Systems involving the continuous circulation of liquids by airlift have also been used. In this way, there is a continuous circulation of the liquid from the storage tank to the cavity, in which the end of the needle is located. This ensures a good homogenization of the liquid as a result of the circulation and the latter permits an effective rinsing of the pipes. However, at the actual needle, there are still risks of deposits taking place. In addition, airlift systems have been used in which a circulation of the liquid takes place in the actual container, as a result of a sampling device having two needles connected to the liquid supply and discharge tubes. Although this ensures homogenization and rinsing of the circuit, this system only has a limited efficiency level.
Another system also uses a liquid circulation by airlift, but the airlift head has a small tank positioned between the small diameter supply tube and the larger diameter return tube. The liquid arriving by the small tube is discharged into the tank which, once full, overflows into the return tube. An orifice connects the lower part of the intermediate tank to the discharge tube, in order to ensure emptying when the installation is stopped. However, despite the presence of this orifice, unwanted deposits are still possible in the intermediate tank.